At any time, our mental system is involved in many activities which up to a certain extent are carried out in parallel. The density of the ongoing processes has been neglected for a long time as our science has been a victim of conceiving the mind as a linear sequence of successive processing. Already at the time of Wundt, the founder of our science, it was implicitly agreed that the simultaneous processing of several sensory inputs is next to impossible; one needs to switch attention from one sensory input to another and there are processing costs in switching attention. In Die Geschwindigkeit des Gedankens, Wundt (1862) noticed that it takes time to switch attention from the auditory input to the visual one. On the response side, the phenomenon of the Psychological Refractory Period (PRP) has been investigated under the same assumption: Telford (1931) found that if a relatively short interval (0.5 s or less) separates the stimulus (e.g., auditory tone) for one response (e.g., a key press) from the next stimulus for a subsequent response, then the reaction time of the subsequent response increases relative to those with a longer interval (1 s or more) between stimuli. The increase in reaction time implied that there may be a psychological refractory period that is analogous to the refractory period between successive neural impulses.